To link to this article: DOI:10.1007/s10570-011-9528-9
http://dx.doi.org/10.1007/s10570-011-9528-9
This is an author-deposited version published in: http://oatao.univ-toulouse.fr/
Eprints ID: 4898
To cite this version:
Peydecastaing, Jérôme and Vaca-Garcia, Carlos and Borredon, Elisabeth Bi-
acylation of cellulose: determining the relative reactivities of the acetyl and
fatty-acyl moieties. (2011) Cellulose, vol. 18 (n° 4). pp. 1015-1021. ISSN 0969-
0239
Open Archive Toulouse Archive Ouverte (OATAO) OATAO is an open access repository that collects the work of Toulouse researchers and
makes it freely available over the web where possible.
Any correspondence concerning this service should be sent to the repository
administrator: [email protected]
CORE Metadata, citation and similar papers at core.ac.uk
Provided by Open Archive Toulouse Archive Ouverte
1
Bi-acylation of cellulose: determining the relative reactivities of the acetyl and fatty-acyl moieties
J. PEYDECASTAING , C. VACA-GARCIA , E. BORREDON
INRA, UMR 1010, ENSIACET, 4 allée Emile Monso, F-31030, Toulouse Cedex 4, France Université de Toulouse, INPT, UMR 1010, F-Toulouse, France Tel.: +33 5 34 32 35 46; Fax: +33 5 34 32 35 97
Email: [email protected]
Abstract
The global reaction between acetic anhydride and a fatty acid yields, at equilibrium, an
asymmetric acetic-aliphatic anhydride in a medium containing finally: acetic-fatty anhydride,
acetic anhydride, fatty acid, acetic acid and fatty anhydride. No solvent or catalyst was used to
evaluate the impact of the actual reactivity of the anhydrides. The competition between the
formation of acetyl and fatty acyl ester functions was evaluated by determining the ratio of
acetyl/fatty acyl groups grafted on solid cellulose. The influence of temperature, reaction
time, and length of fatty chain on the total degree of substitution and on the ratio of
acetyl/fatty acyl ester functions was investigated. For the first time, a correlation has been
established between esterification and the length of the aliphatic chain of the fatty acid.
Reactivity of the medium decreased with the number of carbons in the fatty acid, raised to the
power 2.37.
Keywords: mixed cellulose esters, mixed anhydrides, acetic-fatty esters
2
1. Introduction
Mixed cellulose esters (MCE), i.e. substituted by different aliphatic chains, are
polymers with interesting properties. In particular, cellulose acetate-propionate (CAP)
and cellulose acetate-butyrate (CAB), are very important industrially. Research on
other mixed acetic-aliphatic cellulose esters has led to the discovery of polyfunctional
materials combining both good mechanical properties and high hydrophobicity (Vaca-
Garcia et al. 1998).
Two step preparations of MCE, with acetylation usually first, followed by the other
substituent, or with two different anhydrides, are common, even in industrial
processes. Alternatively, a reaction mixture can be used comprising one anhydride and
carboxylic acids. Malm and Hiatt (1939) proposed chloroacetic anhydride, plus a fatty
acid with at least 6 C atoms, such as stearic acid, in the presence of an inert, low-
boiling solvent such as methylene chloride and an esterification catalyst such as p-
toluenesulfonic acid.
Another procedure uses trifluoroacetic anhydride as co-reagent, with a mixture of two
different carboxylic acids to form mixed anhydrides in situ (trifluoroacetic-aliphatic or
aliphatic-aliphatic). The former permit fatty acylation without reacting the fluorinated
moiety, whereas the latter can graft either of the aliphatic moieties (Matsuzaki and
Miyata 1967). The same technique has also been used for the synthesis of various
mixed esters of starch (Yang and Montgomery 2008).
The procedure we will consider in this paper consists of the acylation of cellulose with
mixed acetic-aliphatic anhydrides produced in situ by the reaction between acetic
3
anhydride and a carboxylic acid (Vaca-Garcia et al. 1998; Vaca-Garcia and Borredon
1999; Chemeris et al. 2003). In this way, either of the moieties of the asymmetric
anhydride is grafted on the cellulose (Fig. 1).
The esterification of cellulose using this mixture depends on three factors: the relative
proportion of acetylium and acylium ions, the effect of steric hindrance and the
relative activation energies of each reaction.
The aim of this paper is to study the overall reactivity of this mixture on cellulose, and
to compare the impact of the aliphatic chain length (n), of the different fatty acids
used, on cellulose acylation.
2. Experimental
2.1. Chemicals
Acetic anhydride and the carboxylic acids: propionic (C2:0), octanoic (C8:0), capric
(C10:0), lauric (C12:0), myristic (C14:0), palmitic (C16:0), and oleic (C18:1) of 99%+
purity were purchased from Sigma-Aldrich France. Pentadecanoic acid (C15:0) was
used as internal standard (I.S.) in gas chromatography and was obtained from Fluka
France (99% purity). Phosphoric acid, 85% solution in water, was purchased from
Acros France. Alpha-cellulose from Sigma-Aldrich France was the initial biopolymer
(this cellulose was characterized and gave: degree of polymerization = 960, 4%
pentosans, 7% moisture content).
Trimethylsulphonium hydroxide (TMSH) was obtained from Macherey-Nagel France
as a 0.2 mol/L solution in methanol. Tert-butyl methyl ether (MTBE) HPLC grade was
purchased from Scharlau Spain. All chemicals were stored at 4°C prior to use.
4
2.2. Preparation of the mixed acetic-fatty anhydrides
The appropriate amounts (molar ratio 3:2) of fatty acid and acetic anhydride were
added to a glass reactor fitted with a condenser. Reactions were carried out at 100°C
with mechanical stirring at 500 rpm for 1 hour. The reaction media were analyzed by
reversed-phase HPLC (Peydecastaing et al. 2008) and used without purification for the
synthesis of MCE.
2.3. Synthesis of MCE
Reactions were performed in 50 mL reactors fitted with a condenser. 1 g of undried
cellulose (7% moisture) was stirred at 350 rpm into 15 mL of reaction medium without
catalyst at the chosen temperature for the chosen duration. After cooling down to
about 80°C, 20 mL of ethanol were added to precipitate the soluble fraction. Cellulose
esters were separated by filtration over fritted glass and purified by Soxhlet extraction
with ethanol for 8 h. The purified product was then vacuum dried to constant weight at
70°C for at least 24 h.
2.4. DS determination
To our knowledge, there is no single analytical technique that allows the simultaneous
and accurate determination of the DS of long and short substituents in MCE when the
DS is below 0.1. Consequently, we have employed two different techniques. The first
was used to determine the fatty acyl content and the second, to quantify the acetyl
groups. Both analyses are required to evaluate the individual and total DS-values.
5
2.4.1. Determination of the fatty acyl content
The method based on the reaction of cellulose esters with TMSH followed by gas
chromatography (GC) analysis, was used. The conditions were those recommended
for low DS-values according to a previous work (Peydecastaing et al. 2009 a).
Basically, the MCE sample is reacted in a vial with TMSH for 1 hr at 75°C with
MTBE as solvent. Once the sample has cooled down and the solid decanted, the fatty
acid methyl esters (FAME) are quantified by GC and, from this, the concentration of
fatty acyl groups Cf in solution can be determined.
2.4.2. Determination of the acetyl content
This method is based on saponification of the MCE followed by GC analysis.
A precise amount (about 20 mg) of MCE was weighed and introduced into a 2 mL
vial, to which 1 mL of aqueous sodium hydroxide (0.5 N) and 0.5 L of a 0.5 mmol/L
propionic acid (internal standard) solution in water, were added. The vial was
stirred/shaken for 5 hours in a shaking incubator, set at 1200 rpm and 100°C. The
sample was then cooled down, 45 µL of H3PO4 added and the vial shaken again at
1200 rpm for 10 min at room temperature. Once the solid had decanted, a small
sample of the reaction mixture supernatant, was analyzed by GC. Separation was
achieved in a CP-Select CB for FFAP fused silica capillary column (CP7845, Varian)
25 m, 0.32 mm i.d., 0.25 µm film thickness. Helium was used as carrier gas at a flow
rate of 2.6 mL/min. The temperature of the injector was set at 250°C, the FID detector
at 270°C, and the split ratio at 1:100. Oven temperature was programmed as follows:
6
80°C for 1 minute rising to 145°C at a rate of 20°C/min, and then to 250°C, at a rate of
50°C/min, for 2 minutes. This allowed the separation of the acetic and propionic acids
in less than 12 min. From the GC analysis the concentration of acetyl groups C2 in
solution can be determined.
2.4.3. DS calculation
The DS values of the grafted fatty chain (DSf) and of the acetyl chain (DS2) are
calculated as follows:
where:
Cf: concentration of the FAME in MTBE determined by GC (in mol/L)
Vf: volume of internal standard (pentadecanoic acid) solution added to the analyzed sample (in mL)
mf: mass of the MCE sample analyzed in MTBE with TMSH (in g)
Mf: molar mass of the RCOOH fatty acid
C2: concentration of the acetic acid after saponification and acidification determined by GC (in mol/L)
V2: volume of internal standard (propionic acid) solution added to the analyzed sample (in mL)
m2: mass of the MCE sample analyzed in water (in g)
€
DSf =162.14 ×Cf ×Vf
m f −mf
m2
×Cf ×Vf × (Mf −18.02) −C2 ×V2 × 42.03
€
DS2 =162.14 ×C2 ×V2
m2 −Cf ×Vf × (Mf −18.02) −m2
mf
×C2 ×V2 × 42.03
7
3. Results and discussion
3.1. Acylating agent
The mixed anhydrides are reactive molecules that allow the simultaneous reaction of two
acyl moieties onto cellulose. Their reactivity is related to the asymmetry of the molecule.
In acidic medium, the anhydride is protonated and readily cleaved to form an acylium ion
and a carboxylic acid (Fig. 1). In the case of an acetic-fatty anhydride, both types of
acylium ions can be formed.
Moreover, an asymmetric anhydride coexists with its symmetric anhydrides regardless of
the synthetic pathway used to prepare it. This phenomenon has been thoroughly described
by Peydecastaing et al. (2009 b). At the end of the reaction, a mixture of five compounds:
acetic acid, fatty acid, acetic anhydride, fatty anhydride, and acetic-fatty anhydride is
obtained. Under non-catalyzed conditions, only the anhydrides are reactive to cellulose
(Fig. 2). Acetic acid provides the proton to cleave the mixed anhydride and this can be
considered as an auto-catalytic reaction. In order to study the reactivity phenomena on
solid cellulose, a large excess of liquid reaction medium was used to maintain the
concentration of the reactive species relatively constant. In addition, following the study
of Peydecastaing et al. (2009 b), a ratio of 1.5 mol of fatty acid per 1 mol of acetic
anhydride was chosen to make the asymmetric anhydride the most abundant reactive
entity (58%).
3.2. Synthesis of cellulose acetate-oleate
8
As expected, the substitution yield of acetyl and oleoyl moieties increases with
temperature. And because this system has no external catalyst, the DS-values are
relatively low compared to traditional setups using H2SO4 for cellulose esterification in
heterogeneous systems. However, this choice allows the real reactivity of the molecules to
be focused on. For the maximum tested temperature (175°C), the values for the degree of
substitution of oleates (DS18:1) and acetates (DS2) were 3.9x10-4 and 7.1x10-2 respectively.
In other words, 2% of the hydroxyl groups in cellulose have been esterified.
The individual acetyl and oleoyl DS-values obtained at 160°C are around ten and twenty
times greater respectively than those obtained at 80°C. These results highlight the fact that
the acylation reaction is strongly temperature dependent, and we found a linear correlation
between DS-values and 1/T (Fig. 3). The value of the slope is directly related to the
intrinsic reactivity of the moiety concerned. The gradient for acetylation is 170 times
greater than that of oleoylation, and the temperature at which DS=0, can be found by
extrapolation. These points (72°C and 89°C for acetylation and acylation respectively)
represent the temperature at which the reaction, carried out for 1 hr with no external
catalyst, becomes insignificant.
In order to confirm that these DS-values do not originate from residual reagents trapped in
the structure of cellulose, we performed FTIR analysis of the solvent-extracted MCE. No
characteristic carboxylic acid band was observed, however the characteristic ester band
was clearly seen. In addition, the purification method was also validated by impregnating
a sample of cellulose with the acylation mixture at 20°C for 1 hr followed by the same
purification steps. The DS of this sample, determined by the very accurate GC method,
was null (accuracy of the method for the fatty chains is up to 10-5 values of DS, see
Peydecastaing et al. 2009 a).
9
In the range of temperature studied, the proportion of grafted acetyl moieties to acyl
moieties (DS2/DS18:1) decreases from 389 to 187 as temperature increases from 80 to
175°C as shown in Fig. 4, thus at lower temperatures fatty acylation is inhibited. A change
in the composition of the reaction medium cannot account for this since in a previous
article (Peydecastaing et al. 2009 b), it was demonstrated that it remains practically
constant with temperature.
The most likely hypothesis is that the activation energy for the acetylation is lower than
for the acylation by oleoyl moities. At high temperatures (175°C), there is the necessary
energy for acetylation and fatty acylation and none of the reactions is inhibited. The ratio
then reaches a plateau (linearization of the function DS=f(1/T), and here, esterification by
acetyl groups is 200 times higher than by the fatty acyl groups. Such a difference can be
accounted for by steric hindrance of the fatty chain, with the acetyl group more able to
reach hydroxyl functions in the cellulose microfibrils. Moreover, in Fig. 1 the proportion
of acylium ions (path A or B) will depend on the ease of forming the associated
carboxylic acid. Since the pK of fatty acids is slightly higher than that of acetic acid, the
acetylium ion will be more abundant than the fatty acylium ion.
The reaction time, between 1 and 4 hours, has a typical influence on the reaction. The total
DS-value increases rapidly and levels off after 3 hours (at 140°C). The weak reactivity of
cellulose and the low accessibility of OH groups accounts for this.
It is interesting to note that the DS2/DS18:1 ratio remains constant at about 200 over the
whole reaction time range studied (Fig. 5). This means that acetyl and fatty acyl have
different reaction rates but are not interdependent, confirming the hypothesis formulated
above concerning the activation energies. In addition, as the reaction medium is in a state
of equilibrium and the consumption of acetyl groups is greater than oleoyl ones, we would
10
have expected a change in composition with time and then an effect on the ratio of
grafting. However, this does not happen, and the explanation could be that the molar ratio
of the acetyl/fatty acyl groups present in the reaction medium, has only a small impact on
the concentration of the mixed anhydride in the reaction medium. This observation was
made in a previous paper concerning the study of the synthesis of acetic-oleic anhydride
mixtures (Peydecastaing et al. 2009 b).
3.3. Synthesis of cellulose acetate-alkanoates
In the following series of experiments, the temperature and reaction time were kept
constant (140°C, 1 h) and only the nature of the fatty chain was varied, by preparing
mixtures using acetic anhydride and different fatty acids with saturated aliphatic chains
from C8 to C16, at a molar ratio of 1.5 (Table 1).
The steric hindrance evidently diminishes the fatty acylation (Fig. 6), and logically the
DS2/DSf ratio also decreases as the fatty acid chain length increases. This is well
illustrated by the plot of this ratio as a function of the chain length on a logarithmic scale
(Fig. 7). The regression trend is given by the equation DS2/DSf = 5.1/n2.37 for the
conditions studied. Reactivity decreases with the number of carbon atoms, not in a linear
way, nor to the square but to the power 2.37. This trend is valid for short (C2) to long (C8-
16) and even mono-unsaturated (C18:1) side-chains. Other mono- and poly-unsaturated
chains (linoleic, linolenic) must be tested to extend the validity of this equation to other
cellulose esters.
4. Conclusions
11
· The reaction medium obtained from acetic anhydride and a fatty acid contains acetic-
fatty anhydride as the most abundant reactive compound. On reaction with cellulose, it leads
to the formation of mixed cellulose esters bearing acetyl and fatty acyl groups. In the absence
of any external catalyst or cellulose solvent, low DS-values are obtained (DS2 ≥ 0.072; DSf ≥
1.8x10-3 with 8≤f≤18) but this allows the impact of the actual reactivity of the molecules to be
evaluated.
· Precise DS determination was employed to accurately characterize these mixed, low
substituted cellulose esters.
· Straight reactivity revealed an acetyl to fatty acyl ratio varying from 27 to 659. Steric
hindrance and the low pK of acetic acid account for preferential acetylation.
· This ratio is dependent on the nature of the fatty acid and on the reaction temperature.
However, it is not dependent on reaction time, being constant throughout the reaction.
· Reactivity decreases with the number of carbon atoms to the power 2.37. This
equation is valid for short and long and even mono-unsaturated (C18:1) side-chains.
Acknowledgments: The authors would like to thank the LAPEYRE Company (France) for
financial support.
12
Chemeris, M. M., N. P. Musko, et al. (2003). "Synthesis of cellulose esters in a trifluoroacetic acid medium." Efiry Tsellyulozy i Krakhmala: Sintez, Svoistva, Primenenie, Materialy Yubileinoi Vserossiiskoi Nauchno-Tekhnicheskoi Konferentsii s Mezhdunarodnym Uchastiem, 10th, Suzdal, Russian Federation, May 5-8, 2003: 108-115.
Malm, C. J. and G. D. Hiatt (1939). Mixed organic acid esters of cellulose such as mixed cellulose acetate-oleate, -linoleate or -stearate suitable for precipitation and washing with water. US, (Eastman Kodak Co.).
Matsuzaki, K. and T. Miyata (1967). "Synthesis of mixed cellulose esters of a,b-unsaturated carboxylic and acetic acids and their utilization. II. Reaction of mixtures of a,b-unsaturated carboxylic acid and acetic anhydride with cellulose." Kogyo Kagaku Zasshi 70: 770-774.
Peydecastaing, J., C. Vaca-Garcia, et al. (2008). "Quantitative Analysis of Mixtures of Various Linear Anhydrides and Carboxylic Acids." Chromatographia 68: 685-688.
Peydecastaing, J., C. Vaca-Garcia, et al. (2009). "Accurate determination of the degree of substitution of long chain cellulose esters." Cellulose 16(2): 289-297.
Peydecastaing, J., C. Vaca-Garcia, et al. (2009 b). "Consecutive reactions in an oleic acid and acetic anhydride reaction medium." Eur. J. Lipid Sci. Technol. 111: 723-729.
Vaca-Garcia, C. and M. E. Borredon (1999). "Solvent-free fatty acylation of cellulose and lignocellulosic wastes. Part 2. Reactions with fatty acids." Bioresour. Technol. 70(2): 135-142.
Vaca-Garcia, C., S. Thiebaud, et al. (1998). "Cellulose esterification with fatty acids and acetic anhydride in lithium chloride/N,N-dimethylacetamide medium." J. Am. Oil Chem. Soc. Society 75: 315-319.
Yang, B., R. Montgomery (2008). "Preparation and physical properties of starch mixed esters." Starch 60: 146-158.
13
Fatty chain % Acetic acid
% Acetic anhydride
% Fatty acid
% Fatty anhydride % AOA
Octanoic (C8:0) 25.20 17.25 20.18 10.72 23.76 Decanoic (C10:0) 26.76 14.58 22.23 11.03 25.41 Lauric (C12:0) 27.87 15.61 22.15 7.73 26.64
Myristic (C14:0) 29.16 15.79 22.58 6.71 25.75 Palmitic (C16:0) 29.80 17.48 24.10 5.95 22.67
Oleic (C18:1) 35.30 10.74 24.70 6.00 23.30 Table 1 composition of reaction media (molar percentage) obtained at equilibrium from 1.5 molar ratio mixtures of fatty
acids (C8 to C18:1) and acetic anhydride
14
Fig. 1 Acylation of cellulose treated with a mixed anhydride
15
Fig. 2 Acylation of cellulose treated in a reaction medium obtained from acetic anhydride and a fatty acid
16
Fig. 3 Influence of the reaction temperature on DS2 and DS18:1
(1 h, R=1.5)
17
Fig. 4 Influence of the treatment temperature on DS2/DS18:1
(1 h, R=1.5)
18
Fig. 5 Influence of the treatment duration time on the ratio DS2/DS18:1 (R=1.5)
19
Fig.6 Influence of the fatty acid aliphatic chain length on DS2 and DSf. (140°C for 1 h, R=1.5)
20
Fig. 7 Influence of the fatty acid aliphatic chain length on the ratio DS2/DSf . (140°C for 1 h, R=1.5)